Spinal fusion, especially in the lumbar and sacral region is regularly used to correct and stabilize spinal curves and to stabilize the lumbar and sacral spine temporarily while solid spinal fusions develop in the treatment of other spinal abnormalities. A spinal rod system is one of the stabilization systems currently in use. In spinal rod systems, elongated rods are used to bridge across various portions of the spine. Bone screws and coupling devices are used to attach the rods to various portions of the spinal vertebrae. In some situations one end of the elongated rod is anchored in the sacral region of the spine with the other end being anchored in a selected lumbar vertebrae.
When spinal rod systems are anchored in the sacral region, the ability to achieve strong sacral fixation between the sacral and lumbar vertebrae becomes difficult. The anatomical position of the sacrum can cause a 30.degree. or more difference between the angle of implantation of bone screws in the sacral and lumbar vertebrae. When the spinal rod is not properly contoured to a patient's lordotic curve, misalignment occurs between the implanted bone screws and the coupling devices which causes inadequate fixation of the spinal system.
A spinal rod system currently in use is shown in U.S. Pat. No. 5,102,412 to Rogozinski. In the Rogozinski Spinal System, two elongated rods are used with cross-bars extending laterally between the rods to form a quadrilateral construct. Hooks, bone screws or a combination of hooks and screws are attached to the vertebrae, and U-shaped couplers are used to secure the hooks or screws to the spinal rods. The bone screws each have a T-shaped head which is pivotally received in a coupler. The coupler has a U-shaped open back with a recess for receiving either the T-head of the screw or the spinal rod. The bottom of the coupler has an opening for receiving the threaded shank of the T-screw with the screw head fitting between the side walls of the coupler. The side walls also include aligned openings for receiving an elongate bar that extends through the aligned openings in a direction transverse to the recess. The bar has set screws for holding the T-screw and spinal rod in place in the coupler. Couplers are connected to each other through the elongate bar.
When a spinal rod system such as the Rogozinski spinal system is implanted in the sacral region of the spine, a need exists for a bone screw which allows for the variability in angulation found between the sacral and lumbar vertebrae.
A need also exists for bone screws having the ability to pivot in the medial/lateral plane as well as the ability to pivot and lock in the cephalad/caudal plane while maintaining the proper alignment between an implanted bone screw, a coupler and a rod of a spinal fixation system.